The current invention relates to a composition and method of forming a single stage coating that does not utilize Isocyanate functional crosslinking agents. Currently, in the automotive refinish as well as general coatings marketplace, Isocyanate functional crosslinking agents are used in many applications where extreme durability is required. The toughness and chemical resistance of coatings that have been crosslinked by the Isocyanate/Hydroxyl reaction to form Urethane bonds are considered the industry standard for highly durable coatings applications.
In the current environment of regulations intended to reduce the amount of volatile organic compounds (or VOC) that are released into the atmosphere on application of coatings, it is very difficult to formulate coatings compositions that meet the regulatory limits without some kind of crosslinking mechanism that takes place after application. In order to achieve a suitable spray application viscosity (20-30 seconds #4 Ford cup @20.degree. C.) as VOC regulations have mandated higher application non-volatiles it has been necessary to use resins with lower molecular weights and generally lower Tg. Both of these changes generally have the effect of lengthening the dry time of the applied coating since the number of crosslink reactions required to achieve a "dry to handle" condition increases significantly.
Due to the need to increase the number of crosslinking reactions to achieve the "dry to handle" condition, faster reaction rate crosslinking reactions are desirable.
One reaction well known to the art is that of carboxylic acid functionality and carbodiimide functionality. The problem with the carboxylic acid/carbodiimide chemistry is the reaction rate is extremely fast. There is very little time available after these materials are mixed in which the coatings may be applied to the intended substrate. Also, the cost of development and color matching of an automotive refinish coatings systems that employ acid functional backbone resins would be very high.
The present invention relates a chemistry and method for converting a product line, based on active hydrogen functionality, which was originally intended to be crosslinked with Isocyanate functional crosslinking agents, in situ, to carboxylic acid functionality. This carboxylic acid functionality will then readily react with carbodiimide functionality. While the carboxylic acid/carbodiimide reaction is very rapid, the "relay reaction" rate of active hydrogen to carboxylic acid by the ring opening of the anhydride by the active hydrogen group may be controlled by the choice of anhydride and, optionally, the use of catalyst. Also carbodiimide functional materials may be designed specifically to provide desired final properties.